Yes, sampling all the way to the limit is certainly an exercise in diminishing returns, hence my question about "validity". Personally, I would be of the opinion that MTF50 is a good limit for "validity", such that 100% crops still appear reasonably sharp. Beyond that and the amount of information being captured doesn't seem like it would add much. I don't know how to quantify that concept in any better way, though.Lou Jost wrote:Diffraction is not an all-or-none thing, and there is no such thing as diffraction-free pixels. Diffraction is linear with respect to aperture setting, and there is some diffraction blurring even at apertures that are normally considered to be diffraction-free. The more pixels you can put under a subject feature, the better the image, though we do soon each a point of diminishing returns.
Perhaps a discussion of "output" is a way to start. What is the output of the image capture? I would guess that someone doing large prints would like to see reasonable pixel-level detail, perhaps to the level of MTF50 as I proposed above. Certainly a lot less than this and the image would be considered "soft", even though in reality it may contain more information in the overall image. The only way to see this may be to step back from the print significantly so the eye can down-sample and integrate that information into a whole. This is analogous to down-sizing the image, but if you must downsize in order to see the image clearly, does it really make "valid" sense?